At the present time, commercially available diamond for decoration is a colorless and tranparent diamond with little internal defects is selected from those mined in South Africa or Soviet and thus marketed. Natural decorative diamond, among jewels, is on sale in the largest amounts. As an optical part using diamond, there are IR anvil cells, laser window materials, etc. and in any case, a diamond having no absorption of light in the infrared range, called IIa type, is selected from natural gem stones and used. However, the mining amount of color-less and transparent gem stones is very small, thus resulting in problems on stable supply and price.
That is, natural diamond has the disadvantage that a transparent and colorless rough stone is mined in a small amount and is thus expensive. Above all, the output of a rough stone having no absorption of nitrogen in the infrared range is so little as represented by, for example, an output of the above described IIa type of 1-2% based on the whole rough stones, thue resulting in a problem on supply thereof. In particular, a large-sized window material of 8 mm in diameter is hardly available.
Transparent and colorless diamond has first been synthesized in artificial manner in the 1970s. This synthesis was carried out by the temperature gradient method using a Fe-3.about.5 wt % Al alloy as a solvent metal, thus obtaining several diamond crystals with 0.7.about.1.0 carat in the form of brilliant. The resulting diamond was estimated as G to H by the color scale of natural gem (GIA Standard: standard of gem provided by the American Gem Institute)(U.S. Pat. No. 4,073,380). However, such synthetic diamond has not been marketed because the synthesizing cost is higher than the price of diamond as natural gem.
Artificial diamond is ordinarily colord yellow because during synthesis at an ultra-high pressure and high temperature, nitrogen in a solvent is taken in the crystal lattice, but colorless and transparent diamond can be obtained by adding a nitrogen getter to the solvent. As this nitrogen getter, Al is well known as disclosed in, for example, "The Journal of Physical Chemistry", Vol. 75, No. 12 (1971), p 1838. Specifically, U.S. Pat. No. 4,034,066 discloses that colorless and transparent diamond single crystal of gem grade can be obtained by adding 3 to 5% by weight of Al to a Fe solvent.
As an example of other nitrogen getters than Al, it has been reported in "Muki-zaishitsu Kenkyujo Hokoku (Report of Inorganic Material Institute)", No. 39 (1984), page 16-19 that nitrogen in the crystal is removed by adding Ti or Zr to a solvent metal.
However, combinations capable of producing colorless diamond are only 1 Fe--Zr, 2 (Fe--Ni) alloy-Zr and 3 (Ni--Mn) alloy-Zr and no such colorless diamond can be formed with other combinations. In the above described combinations of 1 to 3, only the combination 1 of Fe--Zr is capable of giving one having no absdorption in a wavelength range of at most 500 nm. The presence of the absorption in the wavelength range of at most 500 nm means that nitrogen atoms are not completely removed.
Furthermore, it has already been reported by H. M. Strong and P. H. Cherenko in "J. Chem. Phys. 75 P 1938 (1971)" that in order to grow a colorless crystal without adding such a nitrogen getter, the growth rate should be reduced to at most 60% of the case of growing a yellow crystal of Ib type.
In particular, synthetic colorless and transparent diamond has not been produced on a commercial scale, because of higher synthesis cost thereof than that of natural decorative diamond. The reason therefor is that not only an expensive and special apparatus is required for the synthesis of such diamond, but also in order to obtain a good quality crystal, it is required to largely lower the growth rate, since in the case of adding a nitrogen getter such as Al to a solvent, the solvent is taken or included in the crystal (this phenomenon will hereinafter be referred to as "inclusion") with the increase of the amount of the nitrogen getter added, thus resulting in an inferior crystal. When using Ti or Zr as a nitrogen getter, in particular, carbides such as TiC or ZrC, formed in the solvent during synthesis, are taken in the crystal and accordingly, it is difficult to obtain a complete crystal.
When using Al as a nitrogen getter and uniformly dissolving it in the solvent metal, it is found necessary in order to synthesize colorless and transparent diamond, according to the inventors' experimental results, to add the nitrogen getter in a proportion of at least 4% by weight (about 12% by volume) to the solvent and in this case, the growth rate should be at least 1 mg/hr so as to effect the crystal growth without inclusion thereof. At this growth rate, however, a synthesis time of 200 hours or more is required, for example, for obtaining a crystal of 1 carat (200 mg), resulting in a large production cost.
When adding another nitrogen getter such as Ti or Zr having a larger reactivity with nitrogen than Al uniformly to the solvent, a colorless and transparent crystal can be obtained even in an amount of 2% by volume and the growth rate can largely be decreased, but large amounts of carbides such as TiC or ZrC are included in the crystal and a good quality crystal can hardly be obtained.
It is an object of the present invention to provide a process for producing an artificial synthetic diamond, which is colorless and transparent and has no absorption of nitrogen in the infrared range, and which can be applied to decorative articles and optical articles, in economical and stable manner, whereby the above described disadvantages and problems of the prior art can be solved.
It is another object of the present invention to provide a method for synthesizing a diamond crystal of IIa type, substantially free from impurities, at a static high pressure without lowering the growth rate, i.e. at a growth rate of about 2 times of the prior art in view of the above described problems.
It is a further object of the present invention to provide a process for producing an artificial synthetic diamond, which is colorless and transparent and substantially free from inclusions, in economical and stable manner, whereby the above described problems of the prior art can be solved and use of the artificial synthetic diamond for decorative and optical appliances is rendered possible.
It is a still further object of the present invention to provide a new process for the production of a good quality diamond single crystal which is colorless and transparent, and free from inclusions by adding a nitrogen getter.